The conventional countershaft gear transmissions comprise a plurality of main gears including a highest gear ratio main gear fixed to a main shaft and the remaining gears which are free to rotate on the main shaft wherein a desired one of the main gears falling into the latter group is selectively coupled with or engaged with the main shaft to establish a power flow path in a desired gear ratio. For shifting between the adjacent gear ratios in the countershaft gear transmission, a clutch, operatively disposed between engine crankshaft and the input shaft of the transmission, has to be disengaged to permit shifting operation in the transmission with the input shaft being subjected to no torque stress.
Under vehicle operating conditions with high loads, including acceleration, a power transmission element following a transmission, such as a propeller shaft, is twisted by an output torque from the transmission before a clutch is disengaged so that when the clutch is disengaged, the torsion is removed to permit the propeller shaft to suddenly return to its normal state together with a countershaft and the like which have a large inertia, thereby to jolt the vehicle, degrading riding quality of the vehicle. This tendency becomes perceivable particularly when a shift is made from the lowest gear ratio, which provides a large speed reduction, to the next gear ratio which also has a large speed reduction, since under this operating condition an output torque from a transmission is considerably large, thus requiring the development of various countermeasures.